Three previously reported non-aqueous electrolyte solutions were investigated electrochemically to determine the kinetics of the Mg deposition–dissolution process as well as the resulting morphology after Mg-ion reduction onto a bulk metal working electrode. Of the solutions examined, the 1.2 M ((CF3)2CH3)COMgCl and 0.2 M AlCl3 in THF solution (F6-t-butoxide) shows the lowest Tafel slope of 26.4 mV/dec, followed by 0.5 M RPhOMgCl and 0.25 M AlCl3 where R = 2,4,6-Me3 in THF (2,4,6-Me3 phenolate, 32.1 mV/dec) and 0.4 M PhMgCl and 0.2 M AlCl3 in THF (APC, 56.2 mV/dec). The fluorinated alkoxide results in complete magnesium coverage of the working electrode, with the metal growing along the [100] direction, orthogonal to the electrode surface. This behavior is contrary to the aromatic-based electrolyte solutions, which show less crystalline Mg deposits and incomplete surface coverage. Through a combination of electron microscopy, X-ray diffraction, and electrochemical methods, we show that this sporadic deposition, in addition to lower solution conductivities, drastically hinders the current density observed for phenolate and APC electrolytes. Electrochemical impedance spectroscopy demonstrates that the largest resistance originates at the platinum–magnesium interface and the solution resistance itself, rather than the actual charge transfer between the electrode and magnesium ions in solution. The facile kinetics of Mg-deposition, along with a strong dependence on surface crystallinity and coverage, suggests the importance of measuring solution conductivity in addition to fully characterizing the resulting magnesium deposits under chronopotentiometric conditions.

A series of solutions containing PhMgCl in THF solvent are examined electrochemically to investigate the adsorption of an electron-insulating layer. Solutions containing no additional salt, as well as those containing added MgCl2 or Al(OPh)3, show an initial anodic current response on a platinum working electrode poised between 2 and 4 V (vs Mg2+/0) followed by electrode passivation (minimal current on continued cycling) due to adsorption of aromatic polymer decomposition products on the platinum. On the other hand, PhMgCl–AlCl3 solutions do not demonstrate the adsorption of an aromatic species and show continuous electrolyte degradation. Once an adsorbed layer is formed, a significant increase in electrode impedance is observed (103 to 105 Ω) with no additional growth of the insulating film. A phenyl radical (Ph•) is deemed the culprit, as adding a phenyl anion source (Ph–) provides apparent 5 V (vs Mg2+/0) stability to nonpassivating electrolyte solutions. Through gel permeation chromatography, the adsorbed polymeric species has an average molar mass of either 50–300 g/mol or 200–800 g/mol depending on electrolysis conditions and electrode composition. Overall, this work shows that electrolytes containing phenyl constituents are nonideal for testing potential rechargeable Mg-ion battery components.

Titanium niobium oxynitrides (TiNbON) are an attractive category of potential photocatalysts, but strategies for preparing them remain limited. We adapt the wet chemical “urea glass” method for pure transition metal nitrides to single-phase mixed-metal titanium niobium nitrides for a range of niobium mole fractions. We then oxidize the nitrides by heating in air to prepare titanium niobium oxynitride that absorbs visible light of λ ≤ 550 nm. The materials are characterized by powder X-ray diffraction, scanning electron microscopy, and diffuse reflectance UV-vis spectroscopy. Their photochemical activity as a function of Nb fraction is benchmarked with methylene blue photomineralization promoted by full-spectrum AM 1.5G solar irradiation with and without a λ ≥ 400 nm cut-on filter. First-order Langmuir–Hinshelwood rate constants for photomineralization reveal a composition with ∼8% Nb to have superior reactivity. Full compositional analysis by Kjeldahl chemical nitrogen determination and energy-dispersive X-ray spectroscopy yields a chemical formula of Ti0.92Nb0.08O1.97N0.03. Finally, electron paramagnetic resonance spectroscopy correlates a localized Nb4+ defect with increased photochemical reaction rate.

ConspectusPhotoelectrochemical (PEC) cells are an ongoing area of exploration that provide a means of converting solar energy into a storable chemical form (molecular bonds). In particular, using PEC cells to drive the water splitting reaction to obtain H2 could provide a clean and sustainable route to convert solar energy into chemical fuels. Since the discovery of catalytic water splitting on TiO2 photoelectrodes by Fujishima and Honda, significant efforts have been directed toward developing high efficiency metal oxides to use as photocatalysts for this reaction. Improving the efficiency of PEC cells requires developing chemically stable, and highly catalytic anodes for the oxygen-evolution reaction (OER). This water oxidation half reaction requires four protons and four electrons coupling in two bond making steps to form O2, which limits the rate.Our group has accelerated efforts in CuWO4 as a candidate for PEC OER chemistry. Its small band gap of 2.3 eV allows for using visible light to drive OER, and the reaction proceeds with a high degree of chemoselectivity, even in the presence of more kinetically accessible anions such as chloride, which is common to seawater. Furthermore, CuWO4 is a chemically robust material when subjected to the highly oxidizing conditions of PEC OER. The next steps for accelerating research using this (and other), ternary phase oxides, is to move beyond reporting the basic PEC measurements to understanding fundamental chemical reaction mechanisms operative during OER on semiconductor surfaces.In this Account, we outline the process for PEC OER on CuWO4 thin films with emphasis on the chemistry of this reaction, the reaction rate and selectivity (determined by controlled-potential coulometry and oxygen-detection experiments). We discuss key challenges with CuWO4 such as slow kinetics and the presence of an OER-mediating mid-gap state, probed by electrochemical impedance spectroscopy. We propose that this mid-gap state imparts the observed chemoselectivity of OER on CuWO4. We introduce insights into the chemical mechanism of PEC OER on CuWO4 using Tafel analysis of electrochemical polarization. We measure Tafel slopes of ∼161 mV/dec, showing that PEC OER proceeds at a slower rate on CuWO4 than on common electrocatalysts for this reaction. Moreover, the observed photocurrent is independent of the borate buffer concentration, signaling that the buffer plays no role in the rate-determining elementary step of the reaction. Finally, we explore some recent developments in doping this material with Co (a known electrocatalytically active metal) and in coupling it with a transparent manganese phosphate (MnPO) electrocatalyst. We find that introducing Co into the wolframite structure leads to detrimental recombination of photogenerated charge carriers. However, coupling CuWO4 with MnPO increases the photocurrent density. Despite some of these challenges, CuWO4 proves to be a robust, visible light absorbing photoanode that can oxidize water with a high degree of selectivity and is therefore worthy of further exploration. Even if new compositions emerge that show better reactivity, this material serves as an excellent proving ground for the common challenges in developing ternary-phase oxides and other compositionally complex materials.

Correction for ‘Improving the stability and selectivity for the oxygen-evolution reaction on semiconducting WO3 photoelectrodes with a solid-state FeOOH catalyst’ by Charles R. Lhermitte et al., J. Mater. Chem. A, 2016, DOI: 10.1039/c5ta04747a.

WO3 electrodes were synthesized via a sol–gel route followed by the photoelectrochemical deposition of a solid state FeOOH oxygen-evolution catalyst (OEC) to observe its effects on electrode stability and selectivity towards the oxygen evolution reaction (OER). WO3 photoanodes have been reported to degrade in aqueous solutions with pH > 3 due to the material's Arrhenius acidity and the potential formation of reactive peroxide intermediates on the WO3 surface during the course of photoelectrochemical water oxidation. The stability during photoelectrochemical OER of WO3 and WO3–FeOOH photoanodes was measured at 1.23 V vs. RHE at pH 4 and 7 in phosphate-buffered solutions. Additionally, the faradaic efficiencies of the electrodes for OER were measured at pH 4. WO3–FeOOH electrodes demonstrate a 95.9 ± 1.6% faradaic efficiency for OER in pH 4 potassium phosphate buffer at current densities of ∼0.75 mA cm−2 under 200 mW cm−2 AM1.5G illumination and an applied bias of 1.43 V vs. RHE. These experiments demonstrate that adding an FeOOH co-catalyst dramatically improves the stability of the electrodes, selectivity, and rate of OER versus that observed on WO3 films.

A compilation of electrolytes within the standard RPhOMgCl and AlCl3 in a THF electrolyte system with varying alkyl-based steric bulks about the phenol ring were prepared and cycled electrochemically. Most notably, 2,4,6-trimethylphenol-based solutions exhibited high conductivity (2.56 mS cm−1) and compatibility with the Chevrel-phase Mo6S8 intercalation cathode (over 200 h cycle time at C/10 current).

Based on DFT predictions, a series of highly soluble fluorinated alkoxide-based electrolytes were prepared, examined electrochemically, and reversibly cycled. The alcohols react with ethylmagnesium chloride to generate a fluoroalkoxy-magnesium chloride intermediate, which subsequently reacts with aluminum chloride to generate the electrolyte. Solutions starting from a 1,1,1,3,3,3-hexafluoro-2-methylpropan-2-ol precursor exhibit high anodic stability, 3.2 V vs Mg2+/0, and a record 3.5 mS/cm solution conductivity. Excellent galvanostatic cycling and capacity retention (94%) is observed with more than 300 h of cycle time while employing the standard Chevrel phase-Mo6S8 cathode material.Keywords: electrolyte; magnesium battery; non-Grignard; rechargeable battery; solution conductivity

We analyze the stability of the non-heme water oxidation catalyst (WOC), Fe(bpmcn)Cl2 toward oxygen and illumination under nonaqueous and acidic conditions. Fe(bpmcn)Cl2 has been previously used as a C–H activation catalyst, a homogeneous WOC, and as a cocatalyst anchored to WO3 for photoelectrochemical water oxidation. This paper reports that the ligand dissociates at pH 1 with a rate constant k = 19.8(2) × 10–3 min–1, resulting in loss of catalytic activity. The combination of UV–vis experiments, 1H NMR spectroscopy, and cyclic voltammetry confirm free bpmcn and Fe2+ present in solution under acidic conditions. Even under nonaqueous conditions, both oxygen and illumination together show slow oxidation of iron over the course of a few hours, consistent with forming an Fe3+–O2– intermediate as corroborated by resonance-enhanced Raman spectroscopy, with a rate constant of k = 3.03(8) × 10–3 min–1. This finding has implications in both the merits of non-heme iron complexes as WOCs as well as cocatalysts in photoelectrochemical schemes: the decomposition mechanisms may include both anchoring group hydrolysis and instability under illumination.

With high elemental abundance, large volumetric capacity, and dendrite-free metal deposition, magnesium metal anodes offer promise in beyond-lithium-ion batteries. However, the increased charge density associated with the divalent magnesium-ion (Mg2+), relative to lithium-ion (Li+) hinders the ion-insertion and extraction processes within many materials and structures known for lithium-ion cathodes. As a result, many recent investigations incorporate known amounts of water within the electrolyte to provide temporary solvation of the Mg2+, improving diffusion kinetics. Unfortunately with the addition of water, compatibility with magnesium metal anodes disappears due to forming an ion-insulating passivating layer. In this short review, recent advances in solid state cathode materials for rechargeable magnesium-ion batteries are highlighted, with a focus on cathode materials that do not require water contaminated electrolyte solutions for ion insertion and extraction processes.In this short review, we present candidate materials for reversible Mg-battery cathodes that are compatible with magnesium metal in water-free electrolytes. The data suggest that soft, polarizable anions are required for reversible cycling.

We present a perspective on recent developments in modified TiO2 photocatalysts for visible light-driven photochemistry with an emphasis on water splitting. We focus on doped and alloyed TiO2 and in particular address the synergistic effects observed in materials with both transition metal cations and nonmetal anions. Several reports have demonstrated absorption of longer wavelengths (λ = 500–600 nm) by codoped materials compared to the absorption edge of TiO2. We review these advances against the backdrop of well-established doped TiO2 research, suggesting on the basis of compositional analysis and wavelength-resolved measurements of photon conversion efficiency that the increase in visible light absorption is likely due to absorption between defect states rather than true band gap narrowing. We draw a distinction between codoped and co-alloyed materials, stressing the attractive electronic structure of the latter. In highlighting recent literature, data examining the rate of photochemical water splitting or magnitude of anodic current as they depend on the wavelength of incident light are emphasized. Finally, areas for further research are highlighted, particularly in the synthesis of co-alloyed compositions of TiO2.

Thin film electrodes of the orthorhombic form of tin tungstate (α-SnWO4) were prepared using a hydrothermal method to convert thin films of WO3 in aqueous SnCl2. The pH dependence of the growth mechanism was identified by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The XRD patterns show complete conversion of WO3(s) to SnWO4(s) at pH 1, 4, and 7. SEM images reveal a morphology change from sponge-like platelets to sharp nanowires as the pH increases from 1 to 7. The α-SnWO4 thin films were reddish brown in color, and display an indirect band gap of 1.9 eV by diffuse reflectance UV–vis spectroscopy. α-SnWO4 is therefore solar-responsive, and a chopped light linear sweep voltammogram recorded under 100 mW/cm2 AM1.5 simulated solar illumination in a pH 5 0.1 mol/L KPi buffer show a visible light response for photoelectrochemical water oxidation, producing 32 μA/cm2 at 1.23 V vs. RHE.Monoclinic WO3 films were converted to orthorhombic α-SnWO4 by a hydrothermal method. The resulting films show a visible light response for photoelectrochemical water oxidation.

Molecular catalysts help overcome the kinetic limitations of water oxidation and generally result in faster rates for water oxidation than do heterogeneous catalysts. However, molecular catalysts typically function in the dark and therefore require sacrificial oxidants such as Ce4+ or S2O82– to provide the driving force for the reaction. In this Communication, covalently anchoring a phosphonate-derivatized complex, Fe(tebppmcn)Cl2 (1), to WO3 removes the need for a sacrificial oxidant and increases the rate of photoelectrochemical water oxidation on WO3 by 60%. The dual-action catalyst, 1-WO3, also gives rise to increased selectivity for water oxidation in pH 3 Na2SO4 (56% on bare WO3, 79% on 1-WO3). This approach provides promising alternative routes for solar water oxidation.

Using Al(OPh)3 rather than the typical AlCl3 with Grignard reagents affords a Mg-ion electrolyte with a reduced chloride content. A 1:4 Al(OPh)3–PhMgCl mixture gives a magnesium tetraphenylaluminate salt that exhibits anodic stability up to 5 V vs. Mg2+/0 on both platinum and stainless steel working electrodes, and shows much reduced corrosion (pitting) of stainless steel after extended electrolysis at 4.5 V.

A series of non-Grignard Mg-electrolytes with various para-substituents was synthesized starting from commercially-available phenols. More electron-withdrawing substituents shift the anodic stability of the electrolyte by 400 mV. The p-CF3 substituted phenol exhibits the highest stability of 2.9 V vs. Mg2+/0, and cycles reversibly with the Chevrel-phase Mo6S8 Mg-ion cathode.

Spinel-structured lithium manganese oxide (LiMn2O4) has attracted much attention because of its high energy density, low cost, and environmental impact. In this article, structural analysis methods such as powder neutron diffraction (PND), X-ray diffraction (XRD), and high-resolution transmission and scanning electron microscopies (TEM & SEM) reveal the capacity fading mechanism of LiMn2O4 as it relates to the mechanical degradation of the material. Micro-fractures form after the first charge (to 4.45 V vs. Li+/0) of a commercial lithium manganese oxide phase, best represented by the formula LiMn2O3.88. Diffraction methods show that the grain size decreases and multiple phases form after 850 electrochemical cycles at 0.2 C current. The microfractures are directly observed through microscopy studies as particle cracks propagate along the (1 1 1) planes, with clear lattice twisting observed along this direction. Long-term galvanostatic cycling results in increased charge-transfer resistance and capacity loss. Upon preparing samples with controlled oxygen contents, LiMn2O4.03 and LiMn2O3.87, the mechanical failure of the lithium manganese oxide can be correlated to the oxygen vacancies in the materials, providing guidance for better synthesis methods.Keywords: electron microscopy; energy storage; Li-ion batteries; mechanical degradation; neutron diffraction; point defects

Although Li-ion batteries have attracted significant interest due to their higher energy density, lack of high rate performance electrode materials and intrinsic safety issues challenge their commercial applications. Herein, we demonstrate a simple photocatalytic reduction method that simultaneously reduces graphene oxide (GO) and anchors (010)-faceted mesoporous bronze-phase titania (TiO2–B) nanosheets to reduced graphene oxide (RGO) through Ti3+–C bonds. Formation of Ti3+–C bonds during the photocatalytic reduction process was identified using electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) techniques. When cycled between 1–3 V (vs Li+/0), these chemically bonded TiO2–B/RGO hybrid nanostructures show significantly higher Li-ion storage capacities and rate capability compared to bare TiO2–B nanosheets and a physically mixed TiO2–B/RGO composite. In addition, 80% of the initial specific (gravimetric) capacity was retained even after 1000 charge–discharge cycles at a high rate of 40C. The improved electrochemical performance of TiO2–B/RGO nanoarchitectures is attributed to the presence of exposed (010) facets, mesoporosity, and efficient interfacial charge transfer between RGO monolayers and TiO2–B nanosheets.Keywords: anode; lithium ion batteries; nanosheets; reduced graphene oxide; titanium dioxide

A new method was developed to synthesize nanosheet-assembled TiO2–B microflowers for Li-ion batteries. Significantly higher electrochemical performance of these microflowers compared to other TiO2–B nanostructures was attributed to their hierarchical microstructure and exposed (1 0) facets of the individual nanosheets.

The photocatalytic activity of anatase-structured Ti1–(5x/4)NbxO2–y–δNy (x = 0.25, y = 0.02; NbN-25) was examined for water oxidation under UV and visible light irradiation. The semiconductor was prepared by sol–gel processing followed by nitridation in flowing ammonia and exhibits an indirect optical gap of 2.2 eV. Ti1–(5x/4)NbxO2–y–δNy was loaded with RuO2 by an impregnation technique, and optimized conditions reveal that 1 wt % RuO2 generates 16 μmol O2 from water with concomitant IO3– reduction after 3 h of illumination under simulated solar radiation at a flux of 600 mW/cm2 illumination, which corresponds to 6-sun AM1.5G illumination (compared to no detectible O2 without the RuO2 cocatalyst). A series of cut-on filters shows that the catalyst-loaded semiconductor evolves O2 for λ ≤ 515 nm, and a gas-phase mass spectrometry isotope labeling experiment shows that irradiating an iodate solution in H218O in the presence of 1 wt % RuO2 loaded on NbN-25 gives rise to catalytic water oxidation: both 36O2 and 34O2 are observed. It is unclear whether 16O arises from IO3– or surface reconstruction on the photocatalyst, but ICP-AES analysis of the postirradiated solution shows no dissolved metal ions.

Pure-phase CuWO4 photoanodes with 200 nm thickness were produced by spin-casting sol–gel precursors to evaluate their performance as photoelectrodes for water oxidation. The stability of CuWO4 in potassium phosphate (KPi) and potassium borate (KBi) buffers was evaluated as a function of pH and irradiance. CuWO4 photoanodes demonstrate higher stability at pH 3 and 5 in a 0.1 M KPi buffer and are significantly more stable over a 12 h period of illumination in a 0.1 M KBi buffer at pH 7 (∼75 μA/cm2 photocurrent at 1.23 V vs RHE (reversible hydrogen electrode) and 1 sun illumination) than in a 0.1 M KPi buffer at pH 7. The onset of photoelectrochemical water oxidation and electrochemical O2 reduction is dictated by Cu(3dx2–y2) states that reside at 0.4 V vs RHE, determined by linear sweep voltammetry. The onset for water oxidation is hindered by a large charge-transfer resistance, as high as 4.6 kΩ at 1 V vs RHE. Nevertheless, CuWO4 photoanodes show nearly quantitative faradic efficiency for water oxidation, even in the presence of chloride, an improvement over the binary oxide WO3.

Visible-light-absorbing compounds that generate active oxygen species in water are needed to maximize the rate of organic dye degradation under incident solar radiation. In this study, a series of Ti1–(5x/4)NbxO2–y–δNy compounds, herein denoted as TiO2:(Nb,N)-x, with varying mole percentage of niobium substituting for titanium (x = 1–30) was prepared by a sol–gel process followed by nitridation under flowing ammonia. All compositions crystallized in the anatase structure, as determined by powder X-ray diffraction, and diffuse reflectance UV–vis spectroscopy showed that the indirect band gap ranged from 2.37 eV (x = 1) to 2.20 eV (x = 30). X-ray photoelectron spectroscopy revealed that the mole percentage of substitutional nitrogen in the compounds was a linear function of the mole percentage of niobium present. NbN-25 and -30 compounds exhibited a 6-fold increase in the rate of methylene blue dye degradation (k = 0.779 and 0.759 h–1, respectively) compared to lower-mole-percentage niobium samples NbN-1 and -5 (k = 0.115 and 0.146 h–1, respectively).

Electrochemical impedance spectroscopy (EIS) was used to probe the electrode/electrolyte interface of CuWO4 thin films prepared by sol–gel methods for water oxidation under simulated solar irradiation. The presented results indicate that the onset of photocurrent is dictated by the presence of a midgap state that participates in water oxidation. The state is likely composed of Cu(3d) orbitals because of both experimental and theoretical evidence of Cu-based orbitals comprising the top of the valence band and the bottom of the conduction band in the bulk. This midgap state was identified experimentally by electrochemical impedance spectroscopy under simulated solar irradiation in borate buffer at pH 7.00. Our results show the evolution of two-charge-transfer events in the Nyquist and Bode plots of EIS data as well as the Fermi level pinning by Mott–Schottky analysis in the potential range of 0.81–1.01 V (reversible hydrogen electrode, RHE). The Mott–Schottky analysis at low frequencies in the dark suggests that it is not a photogenerated state but rather a permanent state in the electronic structure of CuWO4. The same results are observed in pH 9.24 borate buffer, and the midgap state shows a Nernstian pH response.

Lithium-rich manganospinel (Li1+xMn2–xO4–δ, lithium manganese oxide) has been synthesized by hydrothermal methods employing potassium permanganate, lithium hydroxide, and acetone as synthons. The solid product crystallizes as 30–50 nm particles with some larger 100–300 nm particles also occurring. Materials prepared by this low-temperature route contain oxygen vacancies which can be demonstrated by combining thermogravimetric analysis, differential scanning calorimetry, and cyclic voltammetry. Oxygen vacancies can be minimized beyond the limits of detection for these experiments by annealing the compound in air at 500 °C for 4 h. At room temperature, Rietveld refinement of the powder neutron diffraction pattern shows an orthorhombic Fddd(α00) superlattice of the Fdm space group for hydrothermally synthesized lithium manganospinel. After annealing, oxygen vacancies are eliminated and the superlattice features disappear. Furthermore, the hydrothermal synthesis of lithium manganospinel performed under a pure oxygen atmosphere followed by annealing at 500 °C for 4 h in air gives superior electrochemical properties. This compound shows a reversible capacity of 115 mAh/g when cycled at a rate C/3 and retains 93.6% of this capacity after 100 cycles. This same capacity is observed at the faster rate of 3C. At 5C, the capacity drops to 99 mAh/g, but capacity retention remains greater than 95% after 100 cycles. Finally, when cycled at 5C at an elevated temperature of 55 °C, the O2 annealed sample shows an initial capacity of 99 mAh/g with 89% capacity retention after 100 cycles. The high rate capability of this material is ascribed to fast lithium-ion diffusion, estimated to be 10−7 to 10−9 cm2 s−1 by electrochemical impedance spectroscopy.

Microcrystalline and submicrometer powders of Zn1–xCuxWO4 (0 ≤ x ≤ 1) have been prepared by a solid-state synthesis from stoichiometric quantities of the constituent d-block metal oxide and tungsten oxide as well as from a Pechini sol–gel synthesis starting from the d-block metal nitrate and ammonium metatungstate. The stoichiometry of the product is confirmed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis. X-ray diffraction shows that for the entire range of compositions, a single-phase product crystallizes in the wolframite structure, with a symmetry-lowering transition from P2/c to P1̅ at x = 0.20, concomitant with the first-order Jahn–Teller distortion of Cu2+. Far-IR spectroscopy corroborates that symmetry lowering is directly related to the tetragonal distortion within the CuO6 octahedra, with the Zn–O Au symmetry mode at 320 cm–1 (x = 0) splitting into two stretches at 295 and 338 cm–1 (x = 0.3). UV–vis–NIR spectroscopy shows an optical absorption edge characteristic of an indirect band gap that linearly decreases in energy from 3.0 eV (x = 0) to 2.25 eV (x = 1). SQUID magnetometry shows that Zn1–xCuxWO4 (0.1 ≤ x ≤ 1) has an effective moment of 2.30 ± 0.19 μB per mol copper, typical of Cu2+ in extended solids. For high concentrations of copper (x ≥ 0.8), two transitions are observed: one at high-temperature, 82 K (x = 1.0) that decreases to 59 K (x = 0.8), and the Néel temperature, 23.5 K (x = 1.0) that decreases to 5.5 K (x = 0.8). For x < 0.8, no long-range order is observed. A physical 1:1 mixture of both CuWO4:ZnWO4 shows magnetic ordering identical to that of CuWO4.

A facile two-step hydrothermal method is developed for the large-scale preparation of lithium nickel manganese oxide spinel as a cathode material for lithium ion batteries. In the reaction, nickel is introduced in a first step at neutral pH, followed by lithium insertion under base to form a product having composition Li1.02Ni0.5Mn1.5O3.88. The X-ray diffraction pattern and Raman spectroscopy of the synthesized material support a cubic Fdm structure in which Ni and Mn are disordered on the 16d Wyckoff site, necessary for good cycling characteristics. XP spectroscopy and elemental analysis confirms that Mn remains reduced in the final product (ZMn = 3.82) and that two different chemical environments for Ni exist on the surface. SEM imaging shows a primary particle size of ∼200 nm, and galvanostatic cycling of the material vs. Li+/0 gives a reversible gravimetric capacity of ∼120 mA h g−1 at 1 C rate (147 mA g−1) with reversible cycling up to 1470 mA g−1, supported by rapid Li+ diffusion. The capacity fade at 1 C is substantial, 17.3% over the first 100 cycles between 3.4 and 5.0 V. However, when the voltage limits are altered, the capacity retention is excellent: nearly 100% when cycled either between 3.4 and 4.4 V (where oxygen vacancies are not electrochemically active) or 89% when cycled between 4.4 and 5.0 V (where the Jahn–Teller active Mn4+/3+ couple is not accessed).

Anatase phase TiO2 and related alloyed TiO2:N and TiO2:Nb congeners have been prepared by sol–gel processing techniques. The coalloyed TiO2:(Nb,N)-1 composition, in which niobium substitutes for titanium on the cation sublattice and nitrogen appears in either chemisorbed or interstitial sites as well as substitutes for oxygen on the anion sublattice, has also been prepared. EPR spectroscopy performed on the coalloyed material at 4 K shows that the bulk material contains minor impurities of Ti3+ and F+ centers. Annealing this compound under oxygen oxidizes the material to give TiO2:(Nb,N)-2, which is EPR silent. All alloyed compositions show surface areas of 41–68 m2/g, different from the 2 m2/g for TiO2. In addition, the monoalloyed compounds show band gaps that are not significantly different than that of the parent TiO2 composition (3.2 eV), whereas the coalloyed compound TiO2:(Nb,N)-1 shows a significantly lower energy absorption edge of 2.0 eV. Each composition was tested for its ability to photodegrade methylene blue (MB) dye catalytically, and the coalloyed composition TiO2:(Nb,N)-1 shows a 7-fold increase in rate (1.203 h–1) compared to the parent TiO2 phase. The oxygen annealed version, TiO2:(Nb,N)-2, shows a rate of only 0.234 h–1, leading to the conclusion that bulk Ti3+ and/or F+ centers serve as catalytic sites for MB degradation.

Electrodeposited thin films composed of CuWO4–WO3 oxidize water under AM 1.5G irradiation with no electrical bias and simultaneous reduction of [Fe(CN)6]3– at a Pt-mesh auxiliary electrode with a faradaic efficiency of >85%. The quantum efficiency and apparent quantum yield are 7% (at 400 nm) and 0.038%, respectively, in a representative film. Although low, the photoanode is stable, maintaining its steady-state current density (17 μA/cm2) over a 2.5 h illumination period. Through full photoelectrochemical characterization, we identify the specific drawbacks in our material and propose solutions.

Polycrystalline thin film photoanodes composed of CuWO4 have been prepared by electrodeposition from an acidic aqueous solution. This 2–3 μm thick n-type material has an indirect band gap of 2.25 eV, 0.45 V smaller than the gap of WO3. The flat band potential and donor density determined by Mott–Schottky analysis of the electrochemical impedance spectra are +0.4 V (vs. NHE) and 2.7 × 1021 cm−3 respectively, similar to those observed in crystalline WO3. This implies that the lower band gap is due to a raised valence band maximum, desirable for water oxidation photoanodes. Under neutral conditions afforded by a pH 7 potassium phosphate buffer, electrodeposited CuWO4 generates a photocurrent density of 0.16 mA cm−2 at an applied bias of +0.5 V (vs Ag/AgCl) under simulated solar irradiation. Electrodeposited CuWO4 films demonstrate higher chemical stability than polycrystalline WO3, both under continuous illumination and in the dark. CuWO4 oxidizes methanol and maintains a stable steady-state photocurrent density of 0.11 mA cm−2 after 12 h of continuous illumination in a 10% methanol solution in aqueous electrolyte. This is twice the current density observed without methanol added.

A simple one step hydrothermal route to Li1+xMn2−yO4 spinel compounds (x = 0.01–0.06, y = 0.02–0.09) via reduction of commercially available potassium permanganate with common organic reductants (alcohols, acetone, hex-1-ene, and isobutyraldehyde) in lithium hydroxide aqueous solutions is developed. The cubic spinel phase with no other impurities can be isolated after short reaction times (∼5 h) at the relatively low temperature of 180 °C. Scanning electron microscopy imaging reveals that the crystalline products have a distribution of sizes; the majority of the sample is composed of smaller particles between 10 and 30 nm. However, there are noticeably larger 100–300 nm interspersed particles—more prevalent in reactions with acetone and isobutyraldehyde. In corresponding benchtop test reactions, UV-Vis spectroscopy shows that the disappearance of MnO4− occurs more rapidly when acetone and isobutyraldehyde are used as reducing agents. Cyclic voltammetry performed on our spinels prepared via hydrothermal synthesis shows three reversible redox processes: a wave with E1/2 of ∼2.9 V (vs. Li/Li+) and two close waves between 4.05 and 4.15 V. Galvanostatic cycling of a cell composed of Li1.02Mn1.96O4 prepared from the oxidation of acetone between 3.5 and 4.4 V demonstrates a specific capacity of 104 mA h g−1 on first discharge, with ∼87% capacity retention (90 mA h g−1) after 100 cycles. The specific capacity of all samples correlates with the rate of disappearance of MnO4− observed in our benchtop reactions, providing a facile way to control particle size and electrochemical behavior.

WO3 electrodes were synthesized via a sol–gel route followed by the photoelectrochemical deposition of a solid state FeOOH oxygen-evolution catalyst (OEC) to observe its effects on electrode stability and selectivity towards the oxygen evolution reaction (OER). WO3 photoanodes have been reported to degrade in aqueous solutions with pH > 3 due to the material's Arrhenius acidity and the potential formation of reactive peroxide intermediates on the WO3 surface during the course of photoelectrochemical water oxidation. The stability during photoelectrochemical OER of WO3 and WO3–FeOOH photoanodes was measured at 1.23 V vs. RHE at pH 4 and 7 in phosphate-buffered solutions. Additionally, the faradaic efficiencies of the electrodes for OER were measured at pH 4. WO3–FeOOH electrodes demonstrate a 95.9 ± 1.6% faradaic efficiency for OER in pH 4 potassium phosphate buffer at current densities of ∼0.75 mA cm−2 under 200 mW cm−2 AM1.5G illumination and an applied bias of 1.43 V vs. RHE. These experiments demonstrate that adding an FeOOH co-catalyst dramatically improves the stability of the electrodes, selectivity, and rate of OER versus that observed on WO3 films.

A facile two-step hydrothermal method is developed for the large-scale preparation of lithium nickel manganese oxide spinel as a cathode material for lithium ion batteries. In the reaction, nickel is introduced in a first step at neutral pH, followed by lithium insertion under base to form a product having composition Li1.02Ni0.5Mn1.5O3.88. The X-ray diffraction pattern and Raman spectroscopy of the synthesized material support a cubic Fdm structure in which Ni and Mn are disordered on the 16d Wyckoff site, necessary for good cycling characteristics. XP spectroscopy and elemental analysis confirms that Mn remains reduced in the final product (ZMn = 3.82) and that two different chemical environments for Ni exist on the surface. SEM imaging shows a primary particle size of ∼200 nm, and galvanostatic cycling of the material vs. Li+/0 gives a reversible gravimetric capacity of ∼120 mA h g−1 at 1 C rate (147 mA g−1) with reversible cycling up to 1470 mA g−1, supported by rapid Li+ diffusion. The capacity fade at 1 C is substantial, 17.3% over the first 100 cycles between 3.4 and 5.0 V. However, when the voltage limits are altered, the capacity retention is excellent: nearly 100% when cycled either between 3.4 and 4.4 V (where oxygen vacancies are not electrochemically active) or 89% when cycled between 4.4 and 5.0 V (where the Jahn–Teller active Mn4+/3+ couple is not accessed).

Polycrystalline thin film photoanodes composed of CuWO4 have been prepared by electrodeposition from an acidic aqueous solution. This 2–3 μm thick n-type material has an indirect band gap of 2.25 eV, 0.45 V smaller than the gap of WO3. The flat band potential and donor density determined by Mott–Schottky analysis of the electrochemical impedance spectra are +0.4 V (vs. NHE) and 2.7 × 1021 cm−3 respectively, similar to those observed in crystalline WO3. This implies that the lower band gap is due to a raised valence band maximum, desirable for water oxidation photoanodes. Under neutral conditions afforded by a pH 7 potassium phosphate buffer, electrodeposited CuWO4 generates a photocurrent density of 0.16 mA cm−2 at an applied bias of +0.5 V (vs Ag/AgCl) under simulated solar irradiation. Electrodeposited CuWO4 films demonstrate higher chemical stability than polycrystalline WO3, both under continuous illumination and in the dark. CuWO4 oxidizes methanol and maintains a stable steady-state photocurrent density of 0.11 mA cm−2 after 12 h of continuous illumination in a 10% methanol solution in aqueous electrolyte. This is twice the current density observed without methanol added.

Lithium-rich manganospinel (Li1+xMn2–xO4–δ, lithium manganese oxide) has been synthesized by hydrothermal methods employing potassium permanganate, lithium hydroxide, and acetone as synthons. The solid product crystallizes as 30–50 nm particles with some larger 100–300 nm particles also occurring. Materials prepared by this low-temperature route contain oxygen vacancies which can be demonstrated by combining thermogravimetric analysis, differential scanning calorimetry, and cyclic voltammetry. Oxygen vacancies can be minimized beyond the limits of detection for these experiments by annealing the compound in air at 500 °C for 4 h. At room temperature, Rietveld refinement of the powder neutron diffraction pattern shows an orthorhombic Fddd(α00) superlattice of the Fdm space group for hydrothermally synthesized lithium manganospinel. After annealing, oxygen vacancies are eliminated and the superlattice features disappear. Furthermore, the hydrothermal synthesis of lithium manganospinel performed under a pure oxygen atmosphere followed by annealing at 500 °C for 4 h in air gives superior electrochemical properties. This compound shows a reversible capacity of 115 mAh/g when cycled at a rate C/3 and retains 93.6% of this capacity after 100 cycles. This same capacity is observed at the faster rate of 3C. At 5C, the capacity drops to 99 mAh/g, but capacity retention remains greater than 95% after 100 cycles. Finally, when cycled at 5C at an elevated temperature of 55 °C, the O2 annealed sample shows an initial capacity of 99 mAh/g with 89% capacity retention after 100 cycles. The high rate capability of this material is ascribed to fast lithium-ion diffusion, estimated to be 10−7 to 10−9 cm2 s−1 by electrochemical impedance spectroscopy.

A new method was developed to synthesize nanosheet-assembled TiO2–B microflowers for Li-ion batteries. Significantly higher electrochemical performance of these microflowers compared to other TiO2–B nanostructures was attributed to their hierarchical microstructure and exposed (1 0) facets of the individual nanosheets.

Using Al(OPh)3 rather than the typical AlCl3 with Grignard reagents affords a Mg-ion electrolyte with a reduced chloride content. A 1:4 Al(OPh)3–PhMgCl mixture gives a magnesium tetraphenylaluminate salt that exhibits anodic stability up to 5 V vs. Mg2+/0 on both platinum and stainless steel working electrodes, and shows much reduced corrosion (pitting) of stainless steel after extended electrolysis at 4.5 V.

A compilation of electrolytes within the standard RPhOMgCl and AlCl3 in a THF electrolyte system with varying alkyl-based steric bulks about the phenol ring were prepared and cycled electrochemically. Most notably, 2,4,6-trimethylphenol-based solutions exhibited high conductivity (2.56 mS cm−1) and compatibility with the Chevrel-phase Mo6S8 intercalation cathode (over 200 h cycle time at C/10 current).

Correction for ‘Improving the stability and selectivity for the oxygen-evolution reaction on semiconducting WO3 photoelectrodes with a solid-state FeOOH catalyst’ by Charles R. Lhermitte et al., J. Mater. Chem. A, 2016, DOI: 10.1039/c5ta04747a.

A series of non-Grignard Mg-electrolytes with various para-substituents was synthesized starting from commercially-available phenols. More electron-withdrawing substituents shift the anodic stability of the electrolyte by 400 mV. The p-CF3 substituted phenol exhibits the highest stability of 2.9 V vs. Mg2+/0, and cycles reversibly with the Chevrel-phase Mo6S8 Mg-ion cathode.